Prof. Norton on Molecular Physics. 385 



faces of these figures. We have then first to consider the pro- 

 cess of crystallization as it may occur in a single plane. 



The result of every such process is the arrangement of the 

 molecules in the angular points of a series of quadrilaterals ; 

 which may be squares, rectangles, rhombuses, or rhomboids. If 

 we suppose, in the first instance, several molecules to unite along 

 a single line, and molecules posited on either side of this line to 

 unite with those already crystallized, three different general 

 modes of arrangement may occur : the new particles may take 

 up positions opposite those of the first line, or opposite the mid- 

 dle points of the intervals between these particles, or opposite 

 other than the middle points of these intervals. In the first 

 case, squares or rectangles will be formed ; in the second, rhom- 

 buses, which may in special cases be squares ; and in the third, 

 rhomboids, which in special cases may be rectangles. The ge- 

 neral tendency of the crystallization occurring along the first line 

 should be, by reason of the compression of the molecular atmo- 

 spheres along this line, and the consequent expansion of them 

 in a direction perpendicular to it, to develope an axis of in- 

 creased attraction in this primary line of crystallization, and 

 an axis of diminished attraction in the perpendicular direction. 

 When this result is reached, and successively along the lines of 

 particles parallel to the first, the figure assumed will be either 

 a square, a rectangle, or a rhombus. The two molecular axes 

 will be coincident with the sides of the minute rectangular 

 figures that make up the larger rectangle, and with the diagonals 

 of each minute rhombus. The condition essential to the forma- 

 tion of a square is that the properties of the molecules in refer- 

 ence to cooling (or, in general, in reference to the propagation 

 and absorption of impulses) should be such that each set of four 

 contiguous molecules are, when in the incipient state of crystal- 

 lization, in the same physical condition. That a rectangle may 

 be formed, a group of four particles must unite ; but the escape 

 of the heat-pulses that occurs primarily in the direction of one 

 of the sides of the rectangle must determine a greater compres- 

 sion of the molecular atmospheres in this direction than in that 

 of the other side. That the figure of a rhombus may be assumed, 

 two particles must first unite, and subsequently two other par- 

 ticles must take up, under the attractive action of these, posi- 

 tions opposite the middle of the interval between them. To 

 understand how a rhomboid may result, we must observe that 

 when a line of particles is crystallizing, each particle, m, as it 

 becomes united, exerts a certain disturbing action upon a par- 

 ticle m! next in the line, and also upon a contiguous particle n 

 at one side of the line. When the particle m! unites, it also 

 modifies the condition of n ; but as its action is subsequent to 



